There are many dry materials in fine particulate form which may have a strongly compacting nature, or are strongly binding, or are characterised by a steep angle of repose. These characteristics either singly or combined cause considerable difficulty in discharging these materials from a shipping container as they will not flow easily even when the container is tipped to an angle of say 45.degree..
As used in this specification the term "shipping container" and "container" refer to containers that are primarily used for road, rail, and sea transport and are generally rectangular prismatic in shape.
Examples of fine particulate compactible materials include titanium dioxide, pigments, starch, flour, and many chemicals.
Due to the number of materials falling into this category, the prior art is replete with proposals to facilitate the discharge of these materials from shipping and other forms of storage containers. The following U.S. Patents are relevant:
U.S. Pat. No. 2,943,891 (Paton),
U.S. Pat. No. 3,061,379 (Lusted),
U.S. Pat. No. 2,545,766 (Cline),
U.S. Pat. No. 2,919,955 (Paton),
U.S. Pat. No. 2,968,425 (Paton),
U.S. Pat. No. 3,231,312 (Paton),
U.S. Pat. No. 2,915,337 (Loomis),
U.S. Pat. No. 3,024,072 (Hermanns), and
U.S. Pat. No. 3,375,042 (Ostberg et al.).
In U.S. Pat. No. 2,943,891 (Paton) there is disclosed a fluidizing or unloading floor mat to be placed on the bottom of a storage compartment in order to fluidize materials in small particulate form that are held in the storage compartment and thereby assist the flow of material out of the compartment. The floor mat comprises a bottom layer of gas--impervious material and an upper layer of gas--pervious material with the layers secured together at their periphery to form between them a distributing chamber. Means are provided to introduce gas into the chamber whereby the gas under pressure flows through the gas--pervious layer to fluidize the material stored above the mat. Fluidization of the material assists its discharge.
In order to prevent the upper layer from billowing away from the lower layer, one or more substantially vertical webs may be used to connect the layers together to thereby form a number of compartments within the chamber. In place of webs, the layers may be sewn or glued together along spaced lines.
If all of the webs are provided with holes, a single source of gas may be used. It is, however, taught that when webs without holes are used, they form a series of independent chambers that may be separately supplied with gas. This is said to be desirable as it allows gas to be directed to selected portions thereby allowing for the selective fluidization of material over that portion. It is further disclosed that whilst gas may be directly supplied to the distributing chamber, a preferred means is to provide each of the compartments with a pipe that extends the length of the compartment. The pipe has a series of holes along its length so that gas entering a compartment is evenly distributed. The pipe also serves to keep at least a part of the upper layer apart from the lower layer at all times thereby providing an opening that allows some gas to enter the compartment when gas is first directed thereto.
Similar proposals to U.S. Pat. No. 2,943,891 are made by the same inventor in U.S. Pat. No. 2,919,955, U.S. Pat. No. 2,968,425 and U.S. Pat. No. 3,231,312.
In U.S. Pat. No. 3,061,379 (Lusted) there is disclosed an aerator pad for use in facilitating the discharge of pulverulent materials such as cement from bulk storage. The aerator pad is similar in constructions to the floor mat disclosed in the abovementioned U.S. patent although it is said to be normally limp and flexible thereby allowing the pad to be easily placed into a storage tank. Once in place, air under pressure is supplied to the pad to cause its inflation and then to pass through the gas pervious layer to fluidize the material held in the tank.
In U.S. Pat. No. 2,545,766 (Cline) again fluidization is used to facilitate discharge but in this case, air is only supplied to the leading edge of the material.
In U.S. Pat. No. 2,915,337 (Loomis) it is taught that the bed of material is to be fluidized by an upward flow of gas which is then directed downwardly such that material is entrained in a gas suspension. The gas permeable deck used in this invention is said to be formed from medium woven fabric, porous metal or porous stone.
In U.S. Pat. No. 3,024,072 (Hermanns) there is disclosed an arrangement whereby aerating hoses extend longitudinally over the bottom of a container. At the end remote from the discharge end, a transversely extending gas distribution means for the hoses is provided which is disposed at an angle greater than the angle of repose of the material held in the container. This arrangement is said to improve the discharge of material that otherwise may be retained at the end remote from the discharge end owing to insufficient gas flow in this area.
In U.S. Pat. No. 3,375,042 (Ostberg et al.), the disclosed cargo supporting floor is similar in principle to that disclosed in U.S. Pat. No. 2,943,891 (Paton) with the exception that the air distribution means used to fluidize the cargo comprises inflatable envelopes arranged transversely about a centrally longitudinally extending trough that includes a discharge opening. The envelopes slope towards the trough so that when inflated, material is not only fluidized, but directed downwardly toward the trough.
Although the prior art mentioned above provides means for facilitating the discharge of fine particulate compactible powders from storage containers, there are a number of practical disadvantages which the inventor believes have severely limited their commercial use.
These are:
(a) The fact that the flexible layers lie flat, the one upon the other, during loading, storage, and transport. Given the weight of cargo that lies on the mat, there is considerable resistance to the injection of gas between the layers and to its passage under the cargo. PA1 (b) The existence of ridges between the pockets formed by the inflated mat, in which material is trapped and from which manual discharge is both difficult and costly. PA1 (c) "Dead pockets" which exist at the junction of the flexible fluidizing mat and the container walls in which additional material is trapped (for which discharge is both difficult and costly). This disadvantage can be overcome at some significant cost by extending the fluidizing mat up the sides of the container or by adding extra fluidizing panels to the side of a container liner. However, apart from the disadvantage of the added costs involved, the additional panel will cause creases in the flexible-membrane mat which will inhibit the discharge of the final portion of the cargo. PA1 (d) A major reduction in the effective width of the floor mat when inflated through injection of gas; up to one-third of the width is lost as the flexible membranes used form rounded gas tubes and pull away from the container's side walls. Although it is suggested that a restraining rope can be incorporated in the edge of the flexible fluidizing mat and that this rope can be fastened to the floor and/or side walls of the container to largely overcome this problem, it is not practical. The added costs of the modification and of fastening and detaching the rope are high, and in any case shipping containers conventionally used for road, rail and sea transport do not normally contain suitable fixing points and shipping companies are normally opposed to modifying their general purpose containers in any way. PA1 (e) Pervasive dusting caused by fluidization of fine materials which, for environmental reasons, will normally make desirable the use of a full container liner (with top as well as bottom and sides) to fully enclose the cargo and of filtration equipment to minimize the effects and losses of dust.